The present invention relates to apparatus for automatically focusing an adjustable focus lens, in general, and to apparatus for focusing such a lens in response to a rangefinder derived subject distance signal representative of the optimal distance to a plurality of subjects located at substantially different subject distances, in particular.
Adjustable focus lenses and apparatus employing such lenses are well known and their operation well understood in the art. In addition, the automatic focusing of such lenses in response to a rangefinder derived signal representative of the distance to a particular subject is also well-known in the lens focusing art. A photographic camera having an adjustable focus lens and having a focus control system coupled to said lens for automatically focusing image forming light rays of a subject at the film plane of such a camera in response to an ultrasonic rangefinder derived signal representative of the distance between said camera and the subject to be photographed is sold by Polaroid Corporation under its registered trademark SX-70 SONAR. U.S. Pat. No. 4,199,246 to MUGGLI describes a focus control system coupled to an adjustable focus lens that is responsive to a rangefinder derived signal of the type employed in such a self-focusing camera.
It is well-known that every optical lens system including those of the adjustable focus lens type has a particular depth of field associated therewith or a range of subject distances within which images of subjects located within said depth of field will be in focus at the lens system's focal plane. It is also well-known that the depth of field of an adjustable focus lens is less when the lens is focused on near subjects than it is when focused on those that are more distant, therefore, lens focusing accuracy is significantly more critical when attempting to focus an image of a subject in relatively close proximity to an image forming adjustable focus lens.
In ultrasonic rangefinders of the type employed in the above-mentioned SX-70 SONAR camera, subject distance is determined by determining the distance to the nearest of a group of subjects to be photographed and then causing the camera's lens to sharply focus an image of said nearest subject. Subject distance is primarily determined by determining the round-trip transit time for a burst of ultrasonic energy to reach a particular subject and then be reflected back and subsequently detected by said ultrasonic rangefinder. Subjects located behind or at a greater distance from the rangefinder and beyond the lens' depth of field will be out of sharp focus to a degree dependent upon their particular distance behind said nearest subject. Rangefinders of this type do not have the ability to determine the distance to more remote subjects for lens focusing purposes once a particular nearest subject distance has been determined, even if the number and size of more remote subjects are quite large. As a consequence, and as explained above, these particular more remote subjects will not be in proper focus when there is a narrow depth of field associated with a lens system. This is normally true when the nearest subject is quite close to the rangefinder or lens system.
A primary object of the present invention, therefore, is to provide apparatus for optimizing the image formed by an adjustable focus lens of a plurality of subjects located at substantially different subject distances.
Another object of the present invention is to provide a method and apparatus for automatically focusing the image of a plurality of subjects located at substantially different subject distances by means of an adjustable focus lens system reponsive to a sonic rangefinder derived subject distance signal.
A further object of the present invention is to provide a method and apparatus for minimizing the extent of misfocus of the image of a plurality of subjects located at substantially different subject distances from an adjustable focus lens responsive to a sonic rangefinder derived signal where the rangefinder is of the type that generates a subject distance signal representative of the distance to the nearest of a plurality of such subjects.
Other objects, features, and advantages of the present invention will be readily apparent from the following description of the preferred embodiment thereof taken in conjunction with the accompanying drawings.